1. Field of the Invention
This invention relates to a switching control apparatus, and more particularly to the type of switching control apparatus for intercom-telephone sets.
2. Description of the Prior Art
A conventinal switching device for intercom-telephone sets usually uses relays or transistors to perform the switching operation. When the telephone set is in use and the handset is off the hook, as shown in FIG. 1, contact A of the switch SW1 is connected to contact E, and contact C of the switch SW2 is connected to contact F, and the potential of pin HKC of dial IC A5 of the keyboard A6 goes to a high level so as to drive transistor Q1 to turn on relay RLY1. Meanwhile, current from the telephone line T-R will turn on transistor Q2 via relay RLY1 and flow into the intercom-telephone set through contacts C and F of the switch SW2 coupled with the telephone set for dialing and talking. When talking is ended, pushing key SPK makes the potential of the pin HKC of the dial IC A5 drop to a low level, and transistor Q1 is therefore turned off. As a result, the relay RLY1 returns to its open position, current from telephone lines T-R is cut off, and the intercom function is accomplished.
This switching operation all depends on the relay RLY1. Since the switching of known relays is effected by mechanical contact, instant current passing through the contacts easily generates sparks during on and off operations so that life expectancy and reliability of the relays are greatly affected. Moreover, the known relays can not withstand a high voltage surge. When telephone lines receive roughly a 800 v surge from lightning during electrical storms, the relays will be damaged, resulting in great trouble to the intercom-telephone set and even possibly harm to the user.
Another known switching device for intercom-telephone set is as shown in FIG. 2, a device which has overcome the disadvantage of using relays by substituting transistors. As can be seen in FIG. 2, when the handset is off the hook, current from the telephone lines T-R flows via inverter D1-D4, the switch SW.sub.o, which is closed connecting contact G to contact H, the current flowing into the base of a second transistor Q2, which is turned on, together with a first transistor Q1 so as to energize the talking circuit A2 for effecting dialing and talking through the intercom-telephone set A3. If the handset is not to be held by the user, it has to be positioned in a handset recess (as shown), and the switch SW remains in an open state. When the key SPK of keyboard A6 is pressed, the potential of pin HKC of dial IC A5 goes to a high level so that transistors Q4 and Q3 are turned on in sucession. The current flows from T-R terminal via inverter D1-D4 and passes through transistor Q3 to the speaker-phone circuit A3. Thus the user can talk through the microphone ECM without holding the handset.
When talking is over, the handset is placed on the hook, the key SPK is pusked once again, the pin HKC of dial IC A5 falls to a low level, and Q3, Q4 are turned off. When the switch SW1 is off, Q1 is also cut off, together with the telephone lines T-R. Therefore, the telephone lines T-R remains at break condition with respect to the intercom-telephone set.
Although the transistor circuit provided in the second prior art schematic of FIG. 2 has ameliorated the disadvantage of the relay circuit shown in FIG. 1, disadvantages still remain because, in order to prevent the telephone circuit from being damaged by a lightning surge during electrical storms, special transistors such as those which can resist high voltage have to be used. Thus, the cost of manufacturing for an intercom-telephone set is increased.